My initial impressions after reviewing your photos:
1. I don't understand why the picatinny rail is a seperate piece. Would prefer to see it milled into the upper as one piece. Cost saving measure by purchasing pre-made picatinny rail, but not ideal. For me, it would be easier to machine the upper if it was one piece. You cut the picatinny rail first, then you fixture off that feature for all additional operations. Fixture clamps that hold on picatinny rail are commercially available. It's how I would, and have, done it. Picatinny form cutting mills are readily available, relatively inexpensive, and would not be hard to do. Heck, by eliminating the welding operations, it would probably be cheaper. At the very least, it removes the failure potential of a weld. This is a big "wtf" part of the rifle for me.
2. I would prefer the indexing pin on the handguard was a solid dowel pin, not a spring roll pin. Very minor nitpick.
3. The piston system could be made to come out the front of the gas block. This would mean you don't need to take the handguard off. This would not be hard to do as it appears the plug is already threaded in from the front. I built a few systems like this for my AR180 type rifles.
4. 3D printed parts on a production rifle are still, in my opinion, a full deal breaker.
All relatively small issues in the long run, I guess. I really like the bushing in the upper for the piston rod.
